In South Carolina today, Hillary Clinton scored her biggest victory yet in the Democratic presidential primary. She beat Bernie Sanders by what looks to be nearly 50 percentage points thanks to overwhelming support from African-Americans. As the race heads into Super Tuesday, Clinton has clear momentum: She has big leads in many of the 12 contests that will take place, according to the polls.

According to the South Carolina exit poll, Sanders lost black voters 14 percent to 86 percent. That doomed him in a contest in which 61 percent of voters were black. If white voters were more supportive of his candidacy, Sanders might have been able to keep the race closer. But they split 54 percent for Clinton to 46 percent for Sanders. The split makes the results among white voters in New Hampshire look more like an outlier compared with South Carolina, Iowa and Nevada. Maybe the Vermont senator had more of a next-door-neighbor advantage in New Hampshire than we initially thought.

Perhaps the most worrisome sign for Sanders is that the momentum he had heading into the first three contests seems to have been halted in South Carolina. Sanders was down 25 percentage points in the FiveThirtyEight South Carolina polling average a month ago, and it looks like he’s going to do even worse than that tonight.

Sanders needs something to change because frankly he’s losing.

Indeed, South Carolina is even more of a setback for Sanders than it appears at first glance because it reverses the progress he had been making. If you look at my colleague Nate Silver’s estimates of how Sanders would do in each caucus or primary if the race were tied nationally (Sanders needs to beat these targets to have a shot at the nomination), we see that Sanders did 19 percentage points worse than the benchmark in Iowa, 10 percentage points worse in New Hampshire and 5 percentage points worse in Nevada. That is, Sanders did not hit the target in any of those contests, but he got closer to it as time went on. In South Carolina, it looks like Sanders will run nearly 30 percentage points worse than we would expect given a tie nationally, suggesting that the race has moved in Clinton’s direction since Nevada.

Sanders’s loss of momentum couldn’t have come at a worst time for his campaign. There are six Super Tuesday states (Alabama, Arkansas, Georgia, Tennessee, Texas and Virginia) where black voters made up a larger share of the electorate in 2008 than they did in Iowa, New Hampshire or Nevada this year. That Sanders couldn’t break through with black voters in either Nevada or South Carolina, despite a heavy investment, makes it difficult to believe he will have any more success in these six states, where his campaign hasn’t put in the same effort.

What’s worse for Sanders — of the 865 delegates up for grabs Tuesday, 66 percent come from these six states. An average of polls in each state1 gives Clinton at least a 23 percentage point lead in all of them. These include the two biggest prizes of Super Tuesday: Georgia (102 delegates) where Clinton is up by 39 percentage points and Texas (222 delegates) where Clinton is up by 29 percentage points. If the delegates from these states broke perfectly proportionally based on the polling average, Clinton would end up with a 369 to 202 delegate lead.

In the other six Super Tuesday contests, Sanders has a clear lead in only Vermont, and the candidates are likely to split the delegates in the other five contests fairly evenly. That means that on Super Tuesday, Clinton is likely to win around 508 delegates and Sanders 357.

It’s difficult to oversell how big that lead is. Not only will media be filled with “Clinton Wins Big” headlines, but the way that delegates are awarded in Democratic primaries (proportionally) makes it a tall task to come back from a 100+ delegate deficit. You can’t just win; you have to win big. No one knows this better than Clinton herself; she barely touched Barack Obama’s delegate lead in March and April 2008, even after winning in big states like Ohio, Pennsylvania and Texas. So even if Sanders were to win in states like Wisconsin by a few percentage points, it wouldn’t be enough.

The fact is that South Carolina may spell the beginning of the end of Sanders’s having any real chance of winning more pledged delegates than Clinton. He needs a game-changer between now and Tuesday, or it’ll become a monumental task to catch Clinton in the delegate count.